Jeffrey Andrews-Hanna

This is actually first discovered in a Soviet mission, the first mission to fly past the moon, where the Soviet mission then turned its cameras back to the moon and took a picture and surprised everyone with the fact that the far side of the moon looked nothing like the near side of the moon.

But it's not just in pictures that the moon, that the near side and far side look so different.

This is a topographic map of the moon.

The far side is all high elevation.

The near side is all low elevation.

It is just fundamentally lopsided.

The same thing extends to the thickness of the crust.

The far side of the moon has a very thick crust.

The near side has a very thin crust.

This also extends to composition.

This is a map of the distribution of thorium.

Thorium is one of those radioactive elements that's responsible for driving internal activity for heat production.

and geodynamic activity on the moon, in this map of the thorium, what you see is all the thorium is concentrated on the near side.

Very little of it is on the far side.

So composition, topography, just visible appearance, the far side and near side are fundamentally different, and we fundamentally do not know why.

There really aren't even many good theories to explain this.

My postdoc Alex and I argue about this all the time.

We just had a three-hour heated debate about different theories to try to explain this.

And I can't say we've got any very good ideas.

We have ideas, but not necessarily good ones.